1. Field of the Invention
This invention generally relates to electronic packaging and assembly. More particularly, this invention relates to electronic interconnect technology, specifically for high-frequency and microwave applications.
2. Description of the Related Art
As the operating frequencies of electronic circuits move into the gigahertz range and beyond, the shapes of electronic interconnects become critical to their function. At relatively low frequencies (a "low" frequency is a frequency at which the wavelength of the signal is much longer than the dimensions of the interconnects), simple conductive paths are sufficient and the shapes of those paths have no significant electrical effect. Examples of simple conductive interconnects include: pin-through-hole, wirebonding, tape automated bonding (TAB), leaded surface mount, ball grid array, and flip-chip interconnects.
But at relatively high frequencies, the wavelength is of the same order as, or shorter, than, the dimensions of the interconnects. Simple conductive paths are no longer adequate because of electromagnetic losses and reflections that result from characteristic impedance mismatches between components. This true for both first level (integrated circuit (IC) to package) connections as well as second level (electronic component package to substrate) connections.
One method of connecting components to substrates has been to use surface mount solder interconnects. In general, this process involves placing the electrical contact of an electronic component or substrate, a small amount of solder or solder paste, and a solder wettable pad on a printed circuit board in close proximity. They are then heated until the solder reflows, forming an electrical connection between the solder wettable pad and the electrical contact of the electronic component. Once the solder has cooled, it forms both an electrical and a mechanical connection between the electronic component and the printed circuit board. This process has numerous advantages over other methods of interconnection. First, a large number of components can be interconnected simultaneously. Second, the process is highly repeatable and relatively low cost and is easily adapted for mass production. These interconnections, however, are not generally suited to high-frequency circuits because of electromagnetic leakage and unwanted signal reflections caused by impedance mismatches.
It is known in the art that the high-frequency electrical performance of these interconnects may be improved by surrounding signal interconnects with ground interconnects. For example, a ball grid array signal ball may be surrounded on all sides with ground balls. While this approach improves high-frequency electrical performance, there is still be a considerable amount of electromagnetic leakage between the ground balls. In addition, the characteristic impedance of the interconnect is generally not controlled, potentially causing unwanted signal reflections. The magnitude of both electromagnetic leakage and unwanted signal reflections increase as the operating frequency increases.
In high-frequency applications such as microwave technology in which the electromagnetic leakage and signal reflections must be tightly controlled, interconnects have been made with individually machined coaxial connectors, glass feeds, and semi-rigid coaxial cable. See, for example, U.S. Pat. No. 5,618,205 (Wideband solderless right-angle RF interconnect); U.S. Pat. No. 5,580,276 (Coaxial plug connector component for connection to printed circuit board); U.S. Pat. No. 4,964,805 (Microcoaxial connector having bipartite outer shell); U.S. Pat. No. 4,631,505 (Right angle microwave stripline circuit connector); U.S. Pat. No. 4,577,923 (Microwave integrated circuit and mounting device therefor); and U.S. Pat. No. 4,466,160 (Surface mount type receptacle of coaxial connector and mounting arrangement for mounting receptacle of coaxial connector on substrate). While these types of interconnects have excellent high-frequency electrical characteristics, they consist of many precision made small parts. As a result, they are both expensive to manufacture and not well suited for mass production. Additionally, their relatively large size often prohibits using these interconnects in applications where space is limited.
A first level coaxial interconnect deposited by electroplating and connected through a welding process is also known. See, for example, U.S. Pat. No. 5,347,086 (Coaxial die and substrate bumps). While this type of interconnect has good high-frequency electrical characteristics, it does not meet the need for a low-cost, passively self-aligning electronic interconnect that is easily mass produced.
Accordingly, it is apparent that there is a need for a small, low-cost, passively self-aligning, electronic interconnect that can be mass-produced and assembled with the ease of surface mount solder interconnects while providing the high-frequency electrical performance of individually machined coaxial connectors.